Guarded cover sheet for LCD polarizers and method of making the same

ABSTRACT

The invention generally relates to polymer films used as protective cover sheets for polarizer plates, their manufacture, and to a method for producing polarizing plates employing such polymer films. More particularly, the invention provides a guarded cover sheet composite comprising a temporary carrier substrate having a first cover sheet comprising a first low birefringence film on one side and a second cover sheet comprising a second low birefringence film on the other side.

FIELD OF THE INVENTION

The invention generally relates to polymer films used as protective cover sheets for polarizer plates, their manufacture, and to a method for producing polarizing plates employing such polymer films. More particularly, the invention provides a guarded cover sheet composite comprising a temporary carrier substrate having a first cover sheet comprising a first low birefringence film on one side and a second cover sheet comprising a second low birefringence film on the other side.

BACKGROUND OF THE INVENTION

Transparent resin films are used in a variety of optical applications. In particular, resin films are used as protective cover sheets for light polarizers in variety of electronic displays, particularly Liquid Crystal Displays (LCD).

LCDs contain a number of optical elements that may be formed from resin films. The structure of reflective LCD's may include a liquid crystal cell, one or more polarizer plates, and one or more light management films. Liquid crystal cells are formed by dispersing liquid crystals such as twisted nematic (TN) or super twisted nematic (STN) materials between two electrode substrates. Polarizer plates are typically a multi-layer element of resin films and are comprised of a polarizing film sandwiched between two protective cover sheets. Polarizing films are normally prepared from a transparent and highly uniform amorphous resin film that is subsequently stretched to orient the polymer molecules and stained with a dye to produce a dichroic film. An example of a suitable resin for the formation of polarizer films is fully hydrolyzed polyvinyl alcohol (PVA). Because the stretched PVA films used to form polarizers are very fragile and dimensionally unstable, protective cover sheets are normally laminated to both sides of the PVA film to offer both support and abrasion resistance. Protective cover sheets of polarizer plates are required to have high uniformity, good dimensional and chemical stability, and high transparency. Originally, protective coversheets were formed from glass, but a number of resin films are now used to produce lightweight and flexible polarizers. Although many resins have been suggested for use in protective cover sheets including, cellulosics, acrylics, cyclic olefin polymers, polycarbonates, and sulfones, acetyl cellulose polymers are most commonly used in protective cover sheets for polarizer plates. Polymers of the acetyl cellulose type are commercially available in a variety of molecular weights as well as the degree of acyl substitution of the hydroxyl groups on the cellulose backbone. Of these, the fully substituted polymer, triacetyl cellulose (TAC) is commonly used to manufacture resin films for use in protective cover sheets for polarizer plates.

The cover sheet normally requires a surface treatment to insure good adhesion to the PVA dichroic film. When TAC is used as the protective cover film of a polarizer plate, the TAC film is subjected to treatment in an alkali bath to saponify the TAC surface to provide suitable adhesion to the PVA dichroic film. The alkali treatment uses an aqueous solution containing a hydroxide of an alkali metal, such as sodium hydroxide or potassium hydroxide. After alkali treatment, the cellulose acetate film is typically washed with weak acid solution followed by rinsing with water and drying. This saponification process is both messy and time consuming. U.S. Pat. No. 2,362,580 describes a laminar structure wherein two cellulose ester films each having a surface layer containing cellulose nitrate and a modified PVA is adhered to both sides of a PVA film. JP 06094915A discloses a protective film for polarizer plates wherein the protective film has a hydrophilic layer which provides adhesion to PVA film.

Some LCD devices may contain a protective cover sheet that also serves as a compensation film to improve the viewing angle of an image Compensation films (i.e. retardation films or phase difference films) are normally prepared from amorphous films that have a controlled level of birefringence either by uniaxial stretching or by coating with discotic dyes. Suitable resins suggested for formation of compensation films by stretching include polyvinyl alcohols, polycarbonates and sulfones. Compensation films prepared by treatment with dyes normally require highly transparent films having low birefringence such as TAC and cyclic olefin polymers.

Protective cover sheets may require the application of other functional layers (herein also referred to as auxiliary layers) such as an antiglare layer, antireflection layer, anti-smudge layer, or antistatic layer. Generally, these functional layers are applied in a process step that is separate from the manufacture of the resin film.

In general, resin films are prepared either by melt extrusion methods or by casting methods. Melt extrusion methods involve heating the resin until molten (approximate viscosity on the order of 100,000 cp), and then applying the hot molten polymer to a highly polished metal band or drum with an extrusion die, cooling the film, and finally peeling the film from the metal support. For many reasons, however, films prepared by melt extrusion are generally not suitable for optical applications. Principal among these is the fact that melt extruded films exhibit a high degree of optical birefringence. In the case of highly substituted cellulose acetate, there is the additional problem of melting the polymer. Cellulose triacetate has a very high melting temperature of 270-300° C., and this is above the temperature where decomposition begins. Films have been formed by melt extrusion at lower temperatures by compounding cellulose acetate with various plasticizers as taught in U.S. Pat. No. 5,219,510 to Machell. However, the polymers described in U.S. Pat. No. 5,219,510 to Machell are not the fully substituted cellulose triacetate, but rather have a lesser degree of alkyl substitution or have proprionate groups in place of acetate groups. Even so, melt extruded films of cellulose acetate are known to exhibit poor flatness as noted in U.S. Pat. No. 5,753,140 to Shigenmura. For these reasons, melt extrusion methods are generally not practical for fabricating many resin films including cellulose triacetate films used to prepare protective covers and substrates in electronic displays. Rather, casting methods are generally used to manufacture these films.

Resin films for optical applications are manufactured almost exclusively by casting methods. Casting methods involve first dissolving the polymer in an appropriate solvent to form a dope having a high viscosity on the order of 50,000 cp, and then applying the viscous dope to a continuous highly polished metal band or drum through an extrusion die, partially drying the wet film, peeling the partially dried film from the metal support, and conveying the partially dried film through an oven to more completely remove solvent from the film. Cast films typically have a final dry thickness in the range of 40-200 microns. In general, thin films of less than 40 microns are very difficult to produce by casting methods due to the fragility of wet film during the peeling and drying processes. Films having a thickness of greater than 200 microns are also problematic to manufacture due to difficulties associated with the removal of solvent in the final drying step. Although the dissolution and drying steps of the casting method add complexity and expense, cast films generally have better optical properties when compared to films prepared by melt extrusion methods, and problems associated with decomposition at high temperature are avoided.

Examples of optical films prepared by casting methods include: 1.) Cellulose acetate sheets used to prepare light polarizers as disclosed in U.S. Pat. No. 4,895,769 to Land and U.S. Pat. No. 5,925,289 to Cael as well as more recent disclosures in U.S. patent application Ser. No. 2001/0039319 A1 to Harita and U.S. patent application Ser. No. 2002/001700 A1 to Sanefuji; 2.) Cellulose triacetate sheets used for protective covers for light polarizers as disclosed in U.S. Pat. No. 5,695,694 to Iwata; 3.) Polycarbonate sheets used for protective covers for light polarizers or for retardation plates as disclosed in U.S. Pat. No. 5,818,559 to Yoshida and U.S. Pat. Nos. 5,478,518 and 5,561,180 both to Taketani; and 4.) Polyethersulfone sheets used for protective covers for light polarizers or for retardation plates as disclosed in U.S. Pat. Nos. 5,759,449 and 5,958,305 both to Shiro.

Despite the wide use of the casting method to manufacture optical films, there are however, a number of disadvantages to casting technology. One disadvantage is that cast films have significant optical birefringence. Although films prepared by casting methods have lower birefringence when compared to films prepared by melt extrusion methods, birefringence remains objectionably high. For example, cellulose triacetate films prepared by casting methods exhibit in-plane retardation of 7 nanometers (nm) for light in the visible spectrum as disclosed in U.S. Pat. No. 5,695,694 to Iwata. Polycarbonate films prepared by casting methods exhibit in-plane retardation of 17 nm as disclosed in U.S. Pat. Nos. 5,478,518 and 5,561,180 both to Taketani. U.S. patent application Ser. No. 2001/0039319 A1 to Harita claims that color irregularities in stretched cellulose acetate sheets are reduced when the difference in retardation between widthwise positions within the film is less than 5 nm in the original unstretched film. For many applications of optical films, low in-plane retardation values are desirable. In particular, values of in-plane retardation of less than 10 nm are preferred.

Commonly-assigned U.S. Patent Application Publications 2003/0215658A, 2003/0215621A, 2003/0215608A, 2003/0215583A, 2003/0215582A, 2003/0215581A, 2003/0214715A describe a coating method to prepare resin films having low birefringence that are suitable for optical applications. The resin films are applied onto a discontinuous, sacrificial substrate from lower viscosity polymer solutions than are normally used to prepare cast films. Commonly-assigned U.S. patent application Ser. No. 10/838,841, filed May 4, 2004 describes a guarded protective cover sheet having a removable, carrier substrate and a cover sheet comprising a low birefringence protective polymer film and a layer promoting adhesion to poly(vinyl alcohol) on the same side of the carrier substrate as the low birefringence protective polymer film which eliminates the need for the saponification process.

Birefringence in cast or coated films arises from orientation of polymers during the manufacturing operations. This molecular orientation causes indices of refraction within the plane of the film to be measurably different. In-plane birefringence is the difference between these indices of refraction in perpendicular directions within the plane of the film. The absolute value of birefringence multiplied by the film thickness is defined as in-plane retardation. Therefore, in-plane retardation is a measure of molecular anisotropy within the plane of the film.

During a casting process, molecular orientation may arise from a number of sources including shear of the dope in the die, shear of the dope by the metal support during application, shear of the partially dried film during the peeling step, and shear of the free-standing film during conveyance through the final drying step. These shear forces orient the polymer molecules and ultimately give rise to undesirably high birefringence or retardation values. To minimize shear and obtain the lowest birefringence films, casting processes are typically operated at very low line speeds of 1-15 m/min as disclosed in U.S. Pat. No. 5,695,694 to Iwata. Slower line speeds generally produce the highest quality films.

Another drawback to the casting method is the inability to accurately apply multiple layers. As noted in U.S. Pat. No. 5,256,357 to Hayward, conventional multi-slot casting dies create unacceptably non-uniform films. In particular, line and streak non-uniformity is greater than 5% with prior art devices. Acceptable two layer films may be prepared by employing special die lip designs as taught in U.S. Pat. No. 5,256,357 to Hayward, but the die designs are complex and may be impractical for applying more than two layers simultaneously.

Another drawback to the casting method is the restrictions on the viscosity of the dope. In casting practice, the viscosity of dope is on the order of 50,000 cp. For example, U.S. Pat. No. 5,256,357 to Hayward describes practical casting examples using dopes with a viscosity of 100,000 cp. In general, cast films prepared with lower viscosity dopes are known to produce non-uniform films as noted for example in U.S. Pat. No. 5,695,694 to Iwata. In U.S. Pat. No. 5,695,694 to Iwata, the lowest viscosity dopes used to prepare casting samples are approximately 10,000 cp. At these high viscosity values, however, casting dopes are difficult to filter and degas. While fibers and larger debris may be removed, softer materials such as polymer slugs are more difficult to filter at the high pressures found in dope delivery systems. Particulate and bubble artifacts create conspicuous inclusion defects as well as streaks and may create substantial waste.

In addition, the casting method can be relatively inflexible with respect to product changes. Because casting requires high viscosity dopes, changing product formulations requires extensive down time for cleaning delivery systems to eliminate the possibility of contamination. Particularly problematic are formulation changes involving incompatible polymers and solvents. In fact, formulation changes are so time consuming and expensive with the casting method that most production machines are dedicated exclusively to producing only one film type.

Cast films may exhibit undesirable cockle or wrinkles. In addition, many cast films may naturally become distorted over time due to the effects of moisture. Thinner films are especially vulnerable to dimensional artifacts either during the peeling and drying steps of the casting process or during subsequent handling of the film. In addition, the preparation of polarizer plates requires a lamination process involving saponification pretreatment in an alkali bath and then application of adhesives, pressure, and high temperatures. Very thin, cast cover sheets are difficult to handle during this lamination process without wrinkling. For optical films, good dimensional stability is necessary during storage as well as during subsequent fabrication of polarizer plates. Finally, polymer films used as protective cover sheets for polarizer plates are susceptible to scratch and abrasion, as well as the accumulation of dirt and dust, during the manufacture and handling of the cover sheet.

SUMMARY OF THE INVENTION

The present invention provides a guarded cover sheet composite comprising a temporary carrier substrate having a first cover sheet comprising a first low birefringence film on one side and a second cover sheet comprising a second low birefringence film on the other side. This invention further provides a method of making a guarded cover sheet composite comprising providing a temporary carrier substrate and applying a first coating which will form a first low birefringence film to one side of said carrier substrate; and a second coating which will form a second low birefringence film to the opposite side of said carrier substrate, and drying said coatings. It also provides a method of forming a polarizing plate comprising providing a guarded cover sheet composite comprising a temporary carrier substrate having a first low birefringence cover sheet on one side and a second low birefringence cover sheet on the other side, peeling the first and second cover sheets from the carrier substrate; providing a dichroic film; and simultaneously bringing said first and second cover sheets into adhesive contact with the dichroic film on opposite sides of said dichroic film.

The fabrication of very thin cover sheets is facilitated by the carrier substrate that supports the wet cover sheet coatings through the drying process and eliminates the need to peel the sheets from a metal band or drum prior to a final drying step as required in the casting methods described in prior art. Rather, the cover sheets are completely dried before separation from the carrier substrate. In fact, the composite comprising the cover sheets and carrier substrate are preferably wound into rolls and stored until needed for the fabrication of polarizer plates. The application of two cover sheets on either side of the carrier substrate improves the flatness of the cover sheet composite as well as increasing manufacturing productivity and reducing cost. In addition, cover sheets comprising a layer promoting adhesion to PVA dichoric films eliminates the need for saponification of the cover sheets prior to lamination to the PVA dichroic film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an exemplary coating and drying apparatus that can be used in the practice of the method of the present invention.

FIG. 2 is a schematic of an exemplary coating and drying system that can be used in the practice of the method of the present invention wherein the cover sheets on either side of the carrier substrate are applied sequentially.

FIG. 3 is a schematic of an exemplary coating and drying system that can be used in the practice of the method of the present invention wherein the cover sheet on either side of the carrier substrate are applied simultaneously.

FIG. 4 is a schematic of an exemplary multi-slot coating apparatus that can be used in the practice of the present invention.

FIG. 5 shows a cross-sectional representation of a guarded cover sheet composite of the invention comprising a multi-layer cover sheet on each side of a carrier substrate.

FIG. 6 shows a cross-sectional representation of a guarded cover sheet composite of the invention comprising a multi-layer cover sheet on each side of a carrier substrate wherein each cover sheet comprises a layer promoting adhesion to PVA.

FIG. 7 shows a cross-sectional representation of a guarded cover sheet composite of the invention comprising a multi-layer cover sheet on each side of a carrier substrate wherein the carrier substrate has a release layer formed on each side thereon.

FIG. 8 shows a schematic of a method to fabricate a polarizer plate using the guarded cover sheet composites of the invention.

FIG. 9 shows a cross-sectional representation of a liquid crystal cell with polarizer plates on either side of the cell.

FIG. 10 is a schematic of a casting apparatus as used in prior art to cast cellulose acetate films.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions apply to the description herein:

In-plane phase retardation, R_(in), of a layer is a quantity defined by (nx−ny)d, where nx and ny are indices of refraction in the direction of x and y. x is taken as a direction of maximum index of refraction in the x-y plane and the y direction is perpendicular to it. The x-y plane is parallel to the surface plane of the layer. d is a thickness of the layer in the z-direction. The quantity (nx−ny) is referred to as in-plane birefringence, Δn_(in). The value of Δn_(in) is given at a wavelength λ=550 nm.

Out of-plane phase retardation R_(th), of a layer is a quantity defined by [nz−(nx+ny)/2]d. nz is the index of refraction in the z-direction. The quantity [nz−(nx+ny)/2] is referred to as out-of-plane birefringence, Δn_(th). If nz>(nx+ny)/2, Δn_(th) is positive (positive birefringence), thus the corresponding R_(th) is also positive. If nz<(nx+ny)/2, Δn_(th) is negative (negative birefringence) and R_(th) is also negative. The value of Δn_(th) is given at λ=550 nm.

Intrinsic Birefringence Δn_(int) of a polymer refers to the quantity defined by (ne−no), where ne, and no are the extraordinary and the ordinary index of the polymer, respectively. The actual birefringence (in-plane Δn_(in) or out-of-plane Δn_(th)) of a polymer layer depends on the process of forming it, thus the parameter Δn_(int).

Amorphous means a lack of long-range order. Thus an amorphous polymer does not show long-range order as measured by techniques such as X-ray diffraction.

Transmission is a quantity to measure the optical transmissivity. It is given by the percentile ratio of out coming light intensity I_(out) to input light intensity I_(in) as I_(out)/I_(in)×100.

Optic Axis refers to the direction in which propagating light does not see birefringence.

Uniaxial means that two of the three indices of refraction, nx, ny, and nz, are essentially the same.

Biaxial means that the three indices of refraction, nx, ny, and nz, are all different.

Cover sheets employed in Liquid Crystal Displays are typically polymeric sheets having low optical birefringence that are employed on each side of a dichroic film in order to maintain the dimensional stability of the dichroic film and to protect it from moisture and UV degradation. In the following description a guarded cover sheet means a cover sheet that is disposed on each side of a temporary carrier substrate. The cover sheets on each side of the carrier substrate are peeled from the carrier substrate prior to lamination to a PVA dichroic film. After peeling the cover sheets, the carrier substrate may be discarded, recycled, or reused in the manufacture of another guarded cover sheet composite.

A layer promoting adhesion to PVA is a distinct layer that is applied in a coating step either separate from or simultaneous with the application of the low birefringence polymer film. The layer promoting adhesion to PVA provides acceptable adhesion of the cover sheet to a PVA dichroic film (in a liquid crystal display application) without the need for a wet pretreatment, such as saponification, of the cover sheet prior to lamination to the PVA film.

The present invention is directed to an improved cover sheet used in the fabrication of polarizer plates for Liquid Crystal Displays. In particular, the present invention provides a guarded cover sheet composite comprising a temporary carrier substrate having a first cover sheet comprising a first low birefringence film on one side and a second cover sheet comprising a second low birefringence film on the other side, optionally, one or both cover sheets further comprise a layer promoting adhesion to PVA, and/or one or more auxiliary layers. Suitable auxiliary layers for use in the present invention include abrasion resistant hardcoat layer, antiglare layer, anti-smudge layer or stain-resistant layer, antireflection layer, low reflection layer, antistatic layer, viewing angle compensation layer, tie layer, and moisture barrier layer. The fabrication of very thin cover sheets is facilitated by the carrier substrate that supports the wet cover sheet coatings through the drying process and eliminates the need to peel the sheets from a metal band or drum prior to a final drying step as required in the casting methods described in prior art. Rather, the cover sheets are completely dried before separation from the carrier substrate. The application of two cover sheets on either side of the carrier substrate improves the flatness of the cover sheet composite as well as increasing manufacturing productivity and reducing cost. In addition, cover sheets comprising a layer promoting adhesion to PVA dichroic films eliminates the need for saponification of the cover sheets prior to lamination to the PVA dichroic film.

Turning now to FIG. 1 there is shown a schematic of an exemplary and well-known coating and drying apparatus 10 suitable for preparing the guarded cover sheet composites of the present invention. The coating and drying apparatus 10 is typically used to apply very thin films to a moving carrier substrate 12 and to subsequently remove solvent in a dryer 14. A single coating apparatus 16 is illustrated such that apparatus 10 has only one coating application point and only one dryer 14, however, two or three (even as many as six) additional coating application points with corresponding drying sections are known in the fabrication of composite thin films. The process of sequential application and drying is known in the art as a tandem coating operation. In one embodiment of the present invention, a sequential or tandem coating operation may be employed to prepare a guarded cover sheet composite whereby a coating and drying apparatus as illustrated in FIG. 1 may be used to apply a first cover sheet onto the carrier substrate and then a second coating and drying apparatus as illustrated in FIG. 1 may be used to apply a second cover sheet to the other side of the carrier substrate.

As shown, coating and drying apparatus 10 includes an unwinding station 18 to feed the moving substrate 12 around a back-up roller 20 where the coating is applied by coating apparatus 16. The coated substrate 22 then proceeds through the dryer 14. Dryer 14 will typically use air convection to remove solvent from the coated film. In addition to air convection, dryer 14 may utilize any other means of supplying energy to the coating in order to accelerate the drying rate. Examples of additional energy sources are infra-red or microwave heating elements. An exemplary dryer 14 used in the practice of the method of the present invention includes a first drying section 66 followed by eight additional drying sections 68-82 capable of independent control of temperature and air flow. Although dryer 14 is shown as having nine independent drying sections, drying ovens with fewer compartments are well known and may be used to practice the method of the present invention. In a preferred embodiment of the present invention the dryer 14 has at least two independent drying zones or sections.

Preferably, each of drying sections 68-82 each has independent temperature and airflow controls. In each section, temperature may be adjusted between 5° C. and 150° C. In addition, in each section the airflow characteristics may be altered through the selection of air baffle geometry and the air volumetric flow rate through the baffles. To minimize drying defects from case hardening or skinning-over of the wet layers, optimum drying rates are needed in the early sections of dryer 14 and can be achieved through changes in the air temperature and airflow characteristics. There are a number of artifacts created when temperatures in the early drying zones are inappropriate. For example, fogging or blush of cellulose acetate films is observed when the temperature in zones 66, 68 and 70 are set below 25° C. This blush defect is particularly problematic when high vapor pressures solvents (methylene chloride and acetone) are used in the coating fluids. Aggressively high temperatures above 95° C. or aggressively high air impingement velocities in the early drying sections 66, 68, and 70 are associated with artifacts such as case hardening, mottle, reticulation patterns and blistering of the cover sheet. In preferred embodiment of the present invention, the first drying section 66 is operated at a temperature of at least about 25° C. but less than 95° C. with no direct air impingement on the wet coating of the coated web 22. In another preferred embodiment of the method of the present invention, drying sections 68 and 70 are also operated at a temperature of at least about 25° C. but less than 95° C. It is preferred that initial drying sections 66, 68 be operated at temperatures between about 30° C. and about 60° C. It is most preferred that initial drying sections 66, 68 be operated at temperatures between about 30° C. and about 50° C. The actual drying temperature in drying sections 66, 68 and 70 may optimize empirically within these ranges by those skilled in the art.

As an additional means to prevent blush and fogging defects of cellulose acetate films it is necessary to control the air humidity in drying sections 66-70. In a preferred embodiment of the present invention, the humidity of the air used in drying sections 66-70 is maintained below a dew point temperature of 15° C. It is preferred that the dew point temperature of the air used in drying sections 66-70 be below 10° C. It is most preferred that the dew point temperature of the air used in drying sections 66-70 be below 0° C.

The web path configuration used in drying sections 72-82 is important. Excessive bending of the web can cause premature delamination of the cover sheet from the carrier substrate. In a preferred embodiment of the present invention, the minimum bending radius of curvature during conveyance should be no less than 10 cm. Preferably, the minimum bending radius of curvature during conveyance should be no less than 25 cm. It is most preferred that the web path be flat in drying sections 72-82.

As depicted, an exemplary four-layer coating is applied to moving web 12. Coating liquid for each layer is held in respective coating supply vessel 28, 30, 32, 34. The coating liquid is delivered by pumps 36, 38, 40, 42 from the coating supply vessels to the coating apparatus 16 conduits 44, 46, 48, 50, respectively. In addition, coating and drying apparatus 10 may also include electrical discharge devices, such as corona or glow discharge device 52, or polar charge assist device 54, to modify the substrate 12 prior to application of the coating.

The coating apparatus 16 used to deliver coating fluids to the moving substrate 12 may be a multi-layer applicator such as a slide bead hopper, as taught for example in U.S. Pat. No. 2,761,791 to Russell, or a slide curtain hopper, as taught by U.S. Pat. No. 3,508,947 to Hughes. Alternatively, the coating apparatus 16 may be a multi-manifold extrusion die or a single layer applicator, such as slot die bead hopper, an extrusion die, a wire-wound rod, a knife, an air knife, a blade, a gravure cylinder, a spray nozzle, or jet hopper. In a preferred embodiment of the present invention, the application device 16 is a multi-layer slide bead hopper or a multi-manifold extrusion die.

Turning next to FIG. 2 there is shown a schematic of an exemplary coating and drying system 5 that essentially comprises a tandem of the coating and drying apparatus depicted in FIG. 1. However, coating and drying system 5 further depicts winding station 26 to wind the guarded cover sheet composite 24 into rolls. Accordingly, the drawing in FIG. 2 is numbered in an analogous fashion to the drawing in FIG. 1 up to the winding station, with “a” designating the first coating and drying apparatus and “b” designating the second coating and drying apparatus. In the practice of the present invention the carrier substrate 12 (which may be a resin film, paper, resin coated paper or metal) is supplied from unwind station 18 and conveyed through electrical discharge devices, such as corona or glow discharge device 52 a, or polar charge assist device 54 a, to optionally modify the substrate 12 prior to application of the coating.

A first cover sheet coating solution is applied at coating apparatus 16 a and the coated substrate 22 a then proceeds through the dryer 14 a where the first coating is dried to form cover sheet coated substrate 23. The side of the substrate 23 opposite to that previously coated in coating apparatus 16 a is now conveyed through electrical discharge devices, such as corona or glow discharge device 52 b, or polar charge assist device 54 b, to optionally modify the surface prior to application of second cover sheet coating solution at coating apparatus 16 b. Coated substrate 22 b then proceeds through the dryer 14 b where the second coating is dried to form guarded cover sheet composite 24, which may be wound into rolls at winding station 26.

Turning next to FIG. 3 there is shown a schematic of an exemplary coating and drying system 8 suitable for simultaneously coating and drying a first and second cover sheet on a carrier substrate. Simultaneously coating and drying implies that the both sides of the carrier substrate are coated at the same time or nearly at the same time and then both sides of the substrate are dried at the same time in a single dryer. A suitable coating apparatus to deliver coating fluids to both sides of the moving substrate 12 at the same time is a duplex type coating apparatus as taught by U.S. Pat. No. 5,776,251 to Nobuaki. Alternatively, both sides of the substrate can be coating at the same time using the dip or spray coating methods. Alternatively, both sides of the substrate may be coated nearly at the same time using two closely spaced, but separate, coating apparatus. In this latter case a suitable coating apparatus for the first cover sheet solution is any of the previously mentioned coating apparatuses for single or tandem coatings and a suitable coating apparatus for the second cover sheet solution is any of the previously mentioned coating apparatuses that can be used without a backing roller. Examples of a coating apparatus that can be used without a backing roller and that can instead be operated in a free span of the web path by control of web tension are a single or multiple manifold extrusion die, a blade, a knife, an air knife, a spray nozzle, a wire-wound rod, or a gravure cylinder.

After application of the second cover sheet solution the coated substrate 22 then proceeds through the dryer 14 where both sides of the substrate are dried to form guarded cover sheet composite 24, which may be wound into rolls at winding station 26. It is necessary that a non-contacting web conveyance method be used from the coating application points until the coatings on both sides of the substrate are dried sufficiently so they will not to be damaged by contact. In particular, a vertical web path could be used as illustrated in FIG. 3 without the need for contact until conveyance element 15. The conveyance element in this case could be a roller or an air bar that relies on high air impingement pressure to float the web. In either case it is preferred that the coating that will come into contact with the roller or air bar be dried to at least 50% solids by weight prior to the contact. It is most preferred that the coating be dried to at least 80% solids by weight prior to contacting element 15. It is understood that, in general, dryer 14 could be oriented vertically as shown or in any other direction and that air impingement nozzles can be used within the dryer to accelerate the drying provided that the coating is not damaged.

Referring now to FIG. 4, a schematic of one exemplary coating apparatus 16 is shown in detail. Coating apparatus 16, schematically shown in side elevational cross-section, includes a front section 92, a second section 94, a third section 96, a fourth section 98, and a back plate 100. There is an inlet 102 into second section 94 for supplying coating liquid to first metering slot 104 via pump 106 to thereby form a lowermost layer 108. There is an inlet 110 into third section 96 for supplying coating liquid to second metering slot 112 via pump 114 to form layer 116. There is an inlet 118 into fourth section 98 for supplying coating liquid to metering slot 120 via pump 122 to form layer 124. There is an inlet 126 into back plate 100 for supplying coating liquid to metering slot 128 via pump 130 to form layer 132. Each slot 104, 112, 120, 128 includes a transverse distribution cavity. Front section 92 includes an inclined slide surface 134, and a coating lip 136. There is a second inclined slide surface 138 at the top of second section 94. There is a third inclined slide surface 140 at the top of third section 96. There is a fourth inclined slide surface 142 at the top of fourth section 98. Back plate 100 extends above inclined slide surface 142 to form a back land surface 144. Residing adjacent the coating apparatus or hopper 16 is a coating backing roller 20 about which a web 12 is conveyed. Coating layers 108, 116, 124, 132 form a multi-layer composite which forms a coating bead 146 between lip 136 and substrate 12. Typically, the coating hopper 16 is movable from a non-coating position toward the coating backing roller 20 and into a coating position. Although coating apparatus 16 is shown as having four metering slots, coating dies having a larger number of metering slots (as many as nine or more) are well known and may be used to practice the method of the present invention.

For the purpose of the present invention, the coating fluids are comprised principally of a polymer binder dissolved in an organic solvent. In a particularly preferred embodiment, the low birefringence polymer film (first, second or both) is a cellulose ester. These are commercially available in a variety of molecular weight sizes as well as in the type and degree of alkyl substitution of the hydroxyl groups on the cellulose backbone. Examples of cellulose esters include those having acetyl, propionyl and butyryl groups. Of particular interest is the family of cellulose esters with acetyl substitution known as cellulose acetate. Of these, the fully acetyl substituted cellulose having a combined acetic acid content of approximately 58.0-62.5% is known as triacetyl cellulose (TAC) and is generally preferred for preparing cover sheets used in electronic displays.

In terms of organic solvents for TAC, suitable solvents, for example, include chlorinated solvents (methylene chloride and 1,2 dichloroethane), alcohols (methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, diacetone alcohol and cyclohexanol), ketones (acetone, methylethyl ketone, methylisobutyl ketone, and cyclohexanone), esters (methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, isobutyl acetate, n-butyl acetate, and methylacetoacetate), aromatics (toluene and xylenes) and ethers (1,3-dioxolane, 1,2-dioxolane, 1,3-dioxane, 1,4-dioxane, and 1,5-dioxane). In some applications, small amounts of water may be used. Normally, TAC solutions are prepared with a blend of the aforementioned solvents. Preferred primary solvents include methylene chloride, acetone, methyl acetate, and 1,3-dioxolane. Preferred co-solvents for use with the primary solvents include methanol, ethanol, n-butanol and water.

Coating formulations may also contain plasticizers. Appropriate plasticizers for TAC films include phthalate esters (dimethylphthalate, dimethoxyethyl phthalate, diethylphthalate, dibutylphthalate, dioctylphthalate, didecylphthalate and butyl octylphthalate), adipate esters (dioctyl adipate), and phosphate esters (tricresyl phosphate, biphenylyl diphenyl phosphate, cresyl diphenyl phosphate, octyl diphenyl phospate, tributyl phosphate, and triphenyl phosphate), glycolic acid esters (triacetin, tributyrin, butyl phthalyl butyl glycolate, ethyl phthalyl ethyl glycolate, and methyl phthalyl ethyl glycolate. Plasticizers are normally used to improve the physical and mechanical properties of the final film. In particular, plasticizers are known to improve the flexibility and dimensional stability of cellulose acetate films. However, plasticizers are also used here as coating aids in the converting operation to minimize premature film solidification at the coating hopper and to improve drying characteristics of the wet film. In the method of the present invention, plasticizers are used to minimize blistering, curl and delamination of TAC films during the drying operation. In a preferred embodiment of the present invention, plasticizers are added to the coating fluid at a total concentration of up to 50% by weight relative to the concentration of polymer in order to mitigate defects in the final TAC film.

The coating formulation for the low birefringence polymer may also contain one or more UV absorbing compounds to provide UV filter element performance and/or act as UV stabilizers for the low birefringence polymer film. Ultraviolet absorbing compounds are generally contained in the polymer in an amount of 0.01 to 20 weight parts based on 100 weight parts of the polymer containing no ultraviolet absorber, and preferably contained in an amount of 0.01 to 10 weight parts, especially in an amount of 0.05 to 2 weight parts. Any of the various ultraviolet light absorbing compounds which have been described for use in various polymeric elements may be employed in the polymeric elements of the invention, such as hydroxyphenyl-s-triazine, hydroxyphenylbenzotriazole, formamidine, or benzophenone compounds. As described in copending, commonly assigned U.S. patent application Ser. No. 10/150,634, filed May 5, 2002, the use of dibenzoylmethane ultraviolet absorbing compounds in combination with a second UV absorbing compound such as those listed above have been found to be particularly advantageous with respect to providing both a sharp cut off in absorption between the UV and visible light spectral regions as well as increased protection across more of the UV spectrum. Additional possible UV absorbers which may be employed include salicylate compounds such as 4-t-butylphenylsalicylate; and [2,2′thiobis-(4-t-octylphenolate)]n-butylamine nickel(II). Most preferred are combinations of dibenzoylmethane compounds with hydroxyphenyl-s-triazine or hydroxyphenylbenzotriazole compounds.

Dibenzoylmethane compounds which may be employed include those of the formula (IV)

where R1 through R5 are each independently hydrogen, halogen, nitro, or hydroyxl, or further substituted or unsubstituted alkyl, alkenyl, aryl, alkoxy, acyloxy, ester, carboxyl, alkyl thio, aryl thio, alkyl amine, aryl amine, alkyl nitrile, aryl nitrile, arylsulfonyl, or 5-6 member heterocylce ring groups. Preferably, each of such groups comprises 20 or fewer carbon atoms. Further preferably, R1 through R5 of Formula IV are positioned in accordance with Formula IV-A:

Particularly preferred are compounds of Formula IV-A where R1 and R5 represent alkyl or alkoxy groups of from 1-6 carbon atoms and R2 through R4 represent hydrogen atoms.

Representative compounds of Formula (IV) which may be employed in accordance the elements of the invention include the following:

-   (IV-1): 4-(1,1-dimethylethyl)-4′-methoxydibenzoylmethane (PARSOL®     1789) -   (IV-2): 4-isopropyl dibenzoylmethane (EUSOLEX® 8020) -   (IV-3): dibenzoylmethane (RHODIASTAB® 83)

Hydroxyphenyl-s-triazine compounds which may be used in the elements of the invention, e.g., may be a derivative of tris-aryl-s-triazine compounds as described in U.S. Pat. No. 4,619,956. Such compounds may be represented by Formula V:

wherein X, Y and Z are each aromatic, carbocylic radicals of less than three 6-membered rings, and at least one of X, Y and Z is substituted by a hydroxy group ortho to the point of attachment to the triazine ring; and each of R^(1a) through R^(9a) is selected from the group consisting of hydrogen, hydroxy, alkyl, alkoxy, sulfonic, carboxy, halo, haloalkyl and acylamino. Particularly preferred are hydroxyphenyl-s-triazines of the formula V-A:

wherein R is hydrogen or alkyl of 1-18 carbon atoms.

Hydroxyphenylbenzotriazole compounds which may be used in the elements of the invention, e.g., may be a derivative of compounds represented by Formula VI:

wherein R_(1c) through R_(5c), may be independently hydrogen, halogen, nitro, hydroxy, or further substituted or unsubstituted alkyl, alkenyl, aryl, alkoxy, acyloxy, aryloxy, alkylthio, mono or dialkyl amino, acyl amino, or heterocyclic groups. Specific examples of benzotriazole compounds which may be used in accordance with the invention include 2-(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotriazole; 2-(2′-hydroxy-3′,5′-di-t-amylphenyl)benzotriazole; octyl 5-tert-butyl-3-(5-chloro-2H-benzotriazole-2-yl)-4-hydroxybenzenepropionate; 2-(hydroxy-5-t-octylphenyl)benzotriazole; 2-(2′-hydroxy-5′-methylphenyl)benzotriazole; 2-(2′-hydroxy-3′-dodecyl-5′-methylphenyl)benzotriazole; and 2-(2′-hydroxy-3′,5′-di-t-butylphenyl)-5-chlorobenzotriazole.

Formamidine compounds which may be used in the elements of the invention, e.g., may be a formamidine compound as described in U.S. Pat. No. 4,839,405. Such compounds may be represented by Formula VII or Formula VIII:

wherein R_(1d) is an alkyl group containing 1 to about 5 carbon atoms; Y is a H, OH, Cl or an alkoxy group; R_(2d) is a phenyl group or an alkyl group containing 1 to about 9 carbon atoms; X is selected from the group consisting of H, carboalkoxy, alkoxy, alkyl, dialkylamino and halogen; and Z is selected from the group consisting of H, alkoxy and halogen;

wherein A is —COOR, —COOH, —CONR′R″, —NR′COR, —CN, or a phenyl group; and wherein R is an alkyl group of from 1 to about 8 carbon atoms; R′ and R″ are each independently hydrogen or lower alkyl groups of from 1 to about 4 carbon atoms. Specific examples of formamidine compounds which may be used in accordance with the invention include those described in U.S. Pat. No. 4,839,405, and specifically 4-[[(methylphenylamino)methylene]amino]-ethyl ester.

Benzophenone compounds which may be used in the elements of the invention, e.g., may include 2,2′-dihydroxy-4,4′dimethoxybenzophenone, 2-hydroxy-4-methoxybenzophenone and 2-hydroxy-4-n-dodecyloxybenzophenone.

Coating formulations may also contain surfactants as coating aids to control artifacts related to flow after coating. Artifacts created by flow after coating phenomena include mottle, repellencies, orange-peel (Bernard cells), and edge-withdraw. Surfactants used control flow after coating artifacts include siloxane and fluorochemical compounds. Examples of commercially available surfactants of the siloxane type include: 1.) Polydimethylsiloxanes such as DC200 Fluid from Dow Corning, 2.) Poly(dimethyl, methylphenyl)siloxanes such as DC510 Fluid from Dow Corning, and 3.) Polyalkyl substituted polydimethysiloxanes such as DC190 and DC1248 from Dow Corning as well as the L7000 Silwet series (L7000, L7001, L7004 and L7230) from Union Carbide, and 4.) Polyalkyl substituted poly(dimethyl, methylphenyl)siloxanes such as SF1023 from General Electric. Examples of commercially available fluorochemical surfactants include: 1.) Fluorinated alkyl esters such as the Fluorad series (FC430 and FC431) from the 3M Corporation, 2.) Fluorinated polyoxyethylene ethers such as the Zonyl series (FSN, FSN100, FSO, FSO100) from Du Pont, 3.) Acrylate:polyperfluoroalkyl ethylacrylates such as the F series (F270 and F600) from NOF Corporation, and 4.) Perfluoroalkyl derivatives such as the Surflon series (S383, S393, and S8405) from the Asahi Glass Company. In the method of the present invention, surfactants are generally of the non-ionic type. In a preferred embodiment of the present invention, non-ionic compounds of either the siloxane or fluorinated type are added to the uppermost layers.

In terms of surfactant distribution, surfactants are most effective when present in the uppermost layers of the multi-layer coating. In the uppermost layer, the concentration of surfactant is preferably 0.001-1.000% by weight and most preferably 0.010-0.500%. In addition, lesser amounts of surfactant may be used in the second uppermost layer to minimize diffusion of surfactant into the lowermost layers. The concentration of surfactant in the second uppermost layer is preferably 0.000-0.200% by weight and most preferably between 0.000-0.100% by weight. Because surfactants are only necessary in the uppermost layers, the overall amount of surfactant remaining in the final dried film is small. In the method of the present invention, a practical surfactant concentration in the uppermost layer having a wet thickness of 20 μm and a density of 0.93 g/cc is 0.200% by weight which after drying gives a final surfactant amount of approximately 37 mg/sq-m.

Although surfactants are not required to practice the method of the current invention, surfactants do improve the uniformity of the coated film. In particular, mottle nonuniformities are reduced by the use of surfactants. In transparent cellulose acetate films, mottle nonuniformities are not readily visualized during casual inspection. To visualize mottle artifacts, organic dyes may be added to the uppermost layer to add color to the coated film. For these dyed films, nonuniformities are easy to see and quantify. In this way, effective surfactant types and levels may be selected for optimum film uniformity.

The preparation of the guarded cover sheet composites of the present invention may also include the step of coating over a previously prepared composite of low birefringence polymer film and carrier substrate. For example, the coating and drying apparatus and system shown in FIGS. 1 through 4 may be used to apply a second multi-layer film to an existing low birefringence polymer film/substrate composite. If the film/substrate composite is wound into rolls before applying the subsequent coating, the process is called a multi-pass coating operation. If coating and drying operations are carried out sequentially on a machine with multiple coating stations and drying ovens, then the process is called a tandem coating operation. In this way, thick films may be prepared at high line speeds without the problems associated with the removal of large amounts of solvent from a very thick wet film. Moreover, the practice of multi-pass or tandem coating also has the advantage of minimizing other artifacts such as streak severity, mottle severity, and overall film nonuniformity.

The prior art method of casting resin films is illustrated in FIG. 11. As shown in FIG. 11, a viscous polymeric dope is delivered through a feed line 200 to an extrusion hopper 202 from a pressurized tank 204 by a pump 206. The dope is cast onto a highly polished metal drum 208 located within a first drying section 210 of the drying oven 212. The cast film 214 is allowed to partially dry on the moving drum 208 and is then peeled from the drum 208. The cast film 214 is then conveyed to a final drying section 216 to remove the remaining solvent. The final dried film 218 is then wound into rolls at a wind-up station 220. The prior art cast film typically has a thickness in the range of from 40 to 200 μm.

Coating methods are distinguished from casting methods by the process steps necessary for each technology. These process steps in turn affect a number of tangibles such as fluid viscosity, converting aids, substrates, and hardware that are unique to each method. In general, coating methods involve application of dilute low viscosity liquids to thin flexible substrates, evaporating the solvent in a drying oven, and winding the dried film/substrate composite into rolls. In contrast, casting methods involve applying a concentrated viscous dope to a highly polished metal drum or band, partially drying the wet film on the metal substrate, stripping the partially dried film from the substrate, removing additional solvent from the partially dried film in a drying oven, and winding the dried film into rolls. In terms of viscosity, coating methods require very low viscosity liquids of less than 5,000 cp. In the present invention the viscosity of the coated liquids will generally be less than 2000 cp and most often less than 1500 cp. Moreover, in the present invention the viscosity of the lowermost layer is preferred to be less than 200 cp. and most preferably less than 100 cp. for high speed coating application. In contrast, casting methods require highly concentrated dopes with viscosity on the order of 10,000-100,000 cp for practical operating speeds. In terms of converting aids, coating methods generally involve the use of surfactants as converting aids to control flow after coating artifacts such as mottle, repellencies, orange peel, and edge withdraw. In contrast, casting methods do not require surfactants. Instead, converting aids are only used to assist in the stripping operation in casting methods. For example, n-butanol is sometimes used as a converting aid in casting TAC films to facilitate stripping of the TAC film from the metal drum. In terms of substrates, coating methods generally utilize thin (10-250 μm) flexible supports. In contrast, casting methods employ thick (1-100 mm), continuous, highly polished metal drums or rigid bands. As a result of these differences in process steps, the hardware used in coating is conspicuously different from those used in casting as can be seen by a comparison of the schematics shown, for example, in FIGS. 1 and 10, respectively.

Turning next to FIGS. 5 through 7, there are presented cross-sectional illustrations showing various guarded cover sheet composite configurations possible with the present invention. In FIG. 5, a guarded cover sheet composite 151 comprising two-layer cover sheets 171 and 173 on each side of a carrier substrate 170 is shown. First cover sheet 171 has lowermost layer 161, and outermost layer 165 and second cover sheet 173 has lowermost layer 162 and outermost layer 166. In this illustration, layers 161 and 162 could be a first and a second low birefringence polymer film, respectively, layer 165 could be a viewing angle compensation layer, and layer 166 could be an abrasion resistant hardcoat layer, for example. Typically, layers 161 and 165 may be applied together using a multi-layer slide bead hopper as previously described in FIG. 4. Likewise, layers 162 and 166 may be applied together. Cover sheets 171 and 173 may be formed by simultaneously applying liquid coatings on each side of the carrier substrate 170 and then drying the layers in a single drying operation, or each side of the carrier substrate 170 may be coated and dried in a sequential operation. Alternatively, all four of layers 161, 162, 165, and 166 may be coated and dried in sequential operations.

FIG. 6 illustrates another guarded cover sheet composite 153 comprising a first cover sheet 175 that is comprised of, for example, three compositionally discrete layers including a lowermost layer 161 a nearest to the carrier substrate 170, an intermediate layer 163 a, and an uppermost layer 165 a. On the side of the carrier substrate opposite to first cover sheet 175 is a second cover sheet 177 that is comprised of, for example, two compositionally discrete layers including a lowermost layer 162 a nearest to the carrier substrate 170 and an uppermost layer 166. In this illustration, layers 161 a and 162 a could be a layer promoting adhesion to PVA, layers 163 a and 166 a could be a first and a second low birefringence polymer film, respectively, and layer 165 a could be an abrasion resistant hardcoat layer, for example. The coating and drying operations that may effectively be employed to form guarded cover sheet composite 153 are analogous to those described in FIG. 5 above.

FIG. 7 illustrates a further guarded cover sheet composite 155 comprising multi-layer cover sheets 179 and 181. First cover sheet 179 has lowermost layer 161 b, and outermost layer 165 b and second cover sheet 181 has lowermost layer 162 b, intermediate layer 164 b and outermost layer 166 b. The carrier substrate 182 has been treated on each side with a release layer 184 to modify the adhesion between the substrate 182 and lowermost layers 161 b and 162 b. Release layer 184 may be comprised of a number of polymeric materials such as polyvinylbutyrals, cellulosics, polyacrylates, polycarbonates and poly(acrylonitrile-co-vinylidene chloride-co-acrylic acid). The choice of materials used in the release layer may be optimized empirically by those skilled in the art.

FIGS. 5 through 7 serve to illustrate some of the guarded cover sheet composites that may be constructed based on the detailed teachings provided hereinabove, they are not intended to be exhaustive of all possible variations of the invention. One skilled in the art could conceive of many other layer combinations that would be useful as guarded cover sheet composites for use in the preparation of polarizer plates for LCDs.

Turning now to FIG. 8, a schematic representation of a method to fabricate a polarizer plate from guarded cover sheet composites of the invention is illustrated. Guarded cover sheet composite 153 (see FIG. 6) comprising first cover sheet 175, second cover sheet 177 and carrier substrate 170 is supplied from supply roll 232. A PVA-dichroic film 238 is supplied from supply roll 236. Prior to entering a lamination nip between opposing pinch rollers 242 and 244, the first cover sheet 175 and second cover sheet 177 are peeled from guarded cover sheet composite 153 to expose a lowermost layer (in the case of FIGS. 6, this is lowermost layers 161 a and 162 a, which for the purpose of example are layers promoting adhesion to PVA). The cover sheets may be peeled simultaneously or sequentially, then the bare carrier substrate 170 is wound into a roll at take-up roll 240. A glue solution may be optionally applied to both sides of the PVA-dichroic film or to the lowermost layer of cover sheets 175 and 177 prior to the sheets and film entering the nip between pinch rollers 242 and 244. Cover sheets 175 and 177 are then laminated to either side of PVA-dichroic film 238 with the application of pressure (and, optionally, heat) between the opposing pinch rollers 242 and 244 to give the polarizer plate 250. Polarizer plate 250 may then be dried by heating and wound into rolls until needed. Depending on the particular layer configuration for the guarded cover sheet composite employed, a wide variety of polarizer plates having cover sheets with various combinations of auxiliary layers may be fabricated. Preferably the glue is applied simultaneously with bringing said dichroic film and said cover sheets into contact.

In a preferred embodiment of the invention, the cover sheets are laminated to the PVA dichroic film such that the layer promoting adhesion to PVA is on the side of the cover sheet that contacts the PVA dichroic film. The glue solution useful for laminating the cover film and the PVA dichroic film is not particularly limited, a commonly employed example is a water/alcohol solution containing a dissolved polymer such as PVA or its derivatives and a boron compound such as boric acid. Alternatively, the solution may be free or substantially free of dissolved polymer and comprise a reagent that crosslinks PVA. The reagent may crosslink PVA either ionically or covalently or a combination of both types of reagents may be used. Appropriate crosslinking ions include but are not limited to cations such as calcium, magnesium, barium, strontium, boron, beryllium, aluminum, iron, copper, cobalt, lead, silver, zirconium and zinc ions. Boron compounds such as boric acid and zirconium compounds such as zirconium nitrate or zirconium carbonate are particularly preferred. Examples of covalent crosslinking reagents include polycarboxylic acids or anhydrides; polyamines; epihalohydrins; diepoxides; dialdehydes; diols; carboxylic acid halides, ketenes and like compounds. The amount of the solution applied onto the films can vary widely depending on its composition. For example, a wet film coverage as low as 1 cc/m² and as high as 100 cc/m² are possible. Low wet film coverages are desirable to reduce the drying time needed.

FIG. 9 presents a cross-sectional illustration showing a liquid crystal cell 260 having polarizer plates 252 and 254 disposed on either side. Polarizer plate 254 is on the side of the LCD cell closest to the viewer. Each polarizer plate employs two cover sheets. For the purpose of illustration, polarizer plate 254 is shown with an uppermost cover sheet (this is the cover sheet closest to the viewer) comprising a layer promoting adhesion to PVA 261, low birefringence polymer film 262, moisture barrier layer 264, antistatic layer 266, and antiglare layer 268. The lowermost cover sheet contained in polarizer plate 254 comprises a layer promoting adhesion to PVA 261, low birefringence polymer film 262, moisture barrier layer 264, antistatic layer 266, and viewing angle compensation layer 272. On the opposite side of the LCD cell, polarizer plate 252 is shown with an uppermost cover sheet, which for the purpose of illustration, comprises a layer promoting adhesion to PVA 261, low birefringence polymer film 262, moisture barrier layer 264, antistatic layer 266, and viewing angle compensation layer 272. Polarizer plate 252 also has a lowermost cover sheet comprising a layer promoting adhesion to PVA 261, low birefringence polymer film 262, moisture barrier layer 264, and antistatic layer 266.

Carrier substrates suitable for the use in the present invention include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate, polystyrene, and other polymeric films. Additional substrates may include paper, laminates of paper and polymeric films, glass, cloth, aluminum and other metal supports. Preferably, the carrier substrate is a polyester film comprising polyethylene terephthalate (PET) or polyethylene naphthalate (PEN). The thickness of the carrier substrate is about 10 to 200 micrometers, typically about 10 to 125 micrometers, and preferably 10 to 50 micrometers. Thinner carrier substrates are desirable due to both cost and the weight per roll of guarded cover sheet composite. A particular benefit of the present invention is that by applying a cover sheet on each side of the carrier substrate the curl of the guarded cover sheet composite product is significantly reduced compared to single sided application of a cover sheet onto a carrier substrate as described in commonly-assigned U.S. patent application Ser. No. 10/838,841. However, even in the present invention carrier substrates less than about 10 micrometers may not provide sufficient dimensional stability or protection for the cover sheets.

The carrier substrate may be coated with one or more subbing layers or may be pretreated with electrical discharge devices to enhance the wetting of the substrate by coating solutions. Since the cover sheets must ultimately be peeled from the carrier substrate the adhesion between cover sheets and substrate is an important consideration. Subbing layers and electrical discharge devices may also be employed to modify the adhesion of the cover sheet to the carrier substrate. Subbing layers may therefore function as either primer layers to improve wetting or release layers to modify the adhesion of the cover sheets to the substrate. The carrier substrate may be coated with two subbing layers, the first layer acting as a primer layer to improve wetting and the second layer acting as a release layer. The thickness of the subbing layer is typically 0.05 to 5 micrometers, preferably 0.1 to 1 micrometers.

Cover sheet/substrate composites having poor adhesion might be prone to blister after application of a second or third wet coating in a multi-pass operation. To avoid blister defects, adhesion should be greater than about 0.3 N/m between the first-pass layer of the cover sheet and the carrier substrate. As already mentioned, the level of adhesion may be modified by a variety of web treatments including various subbing layers and various electronic discharge treatments. However, excessive adhesion between the cover sheet and substrate is also undesirable since the cover sheet may be damaged during subsequent peeling operations. In particular, cover sheet/substrate composites having too great an adhesive force may peel poorly. The maximum adhesive force that allows acceptable peel behavior is dependent on the thickness and tensile properties of the cover sheet. Typically, an adhesive force between the cover sheet and the substrate greater than about 300 N/m may peel poorly. Cover sheets peeled from such excessively well-adhered composites exhibit defects due to tearing of the cover sheet and/or due to cohesive failure within the sheet. In a preferred embodiment of the present invention, the adhesion between the cover sheet and the carrier substrate is less than 250 N/m. Most preferably, the adhesion between the cover sheet and the carrier substrate is between 0.5 and 25 N/m.

In a one embodiment of the invention, the carrier substrate is a polyethylene terephthalate film having a first subbing layer (primer layer) comprising a vinylidene chloride copolymer and second subbing layer (release layer) comprising polyvinyl butyral. In another embodiment of the invention the carrier substrate is polyethylene terephthalate film that has been pretreated with a corona discharge prior to application of the cover sheets. In a further embodiment of the invention the cover sheet is applied onto bare and untreated polyethylene terephthalate carrier substrate such that the lowermost layer of each cover sheet is a layer promoting adhesion to PVA.

Substrates may also have functional layers such as antistatic layers containing various polymer binders and conductive addenda in order to control static charging and dirt and dust attraction. The antistatic layer may be on either or both sides of the carrier substrate and also function as a release or subbing layer.

Low birefringence polymer films suitable for use in the present invention comprise polymeric materials having low Intrinsic Birefringence Δn_(int) that form high clarity films with high light transmission (i.e., >85%). Preferably, the low birefringence polymer film has in-plane birefringence, Δn_(in) of less than about 1×10⁻⁴ and an out-of-plane birefringence, Δn_(th) of from 0.005 to −0.005.

Exemplary polymeric materials for use in the low birefringence polymer films of the invention include cellulose esters (including triacetyl cellulose (TAC), cellulose diacetate, cellulose acetate butyrate, cellulose acetate propionate), polycarbonates (such as Lexan® available from General Electric Corp.), polysulfones (such as Udel® available from Amoco Performance Products Inc.), polyacrylates, and cyclic olefin polymers (such as Arton® available from JSR Corp., Zeonex® and Zeonor® available from Nippon Zeon, Topas® supplied by Ticona), among others. Preferably, the low birefringence polymer films of the invention comprises TAC, polycarbonate, or cyclic olefin polymers due to their commercial availability and excellent optical properties.

The low birefringence polymer films have a thickness from about 5 to 100 micrometers, preferably from about 5 to 50 micrometers and most preferably from about 10 to 40 micrometers. Films having thickness of 10 to 40 micrometers are most preferred due to cost, handling, ability to provide thinner polarizer plates, and improved light transmission. Polarizer plates fabricated from conventional cover sheets comprising a low birefringence polymer film thickness of about 80 micrometers have a total thickness of at least 180 micrometers. In a preferred embodiment of the current invention, polarizer plates assembled from cover sheets of the invention have a total thickness of less than 120 micrometers, and most preferably less than 80 micrometers.

In accordance with the present invention the thickness and the composition of the first low birefringence polymer film may be the same or different from the second low birefringence polymer film. The thickness and composition of each low birefringence polymer film will be chosen in order to satisfy the design and performance requirements for the polarizer plate being fabricated and/or provide the needed flatness or freedom from film curl for the guarded cover sheet composite.

Materials useful for forming a layer promoting adhesion to PVA are typically water-swellable, hydrophilic polymers which include both synthetic and natural polymers. Naturally occurring substances include proteins, protein derivatives, cellulose derivatives (e.g. cellulose esters), polysaccharides, casein, and the like, and synthetic polymers include poly(vinyl lactams), acrylamide polymers, polyvinyl alcohol and its derivatives, hydrolyzed polyvinyl acetates, polymers of alkyl and sulfoalkyl acrylates and methacrylates, polyamides, polyvinyl pyridine, acrylic acid polymers, maleic anhydride copolymers, polyalkylene oxide, methacrylamide copolymers, polyvinyl oxazolidinones, maleic acid copolymers, vinyl amine copolymers, methacrylic acid copolymers, acryloyloxyalkyl sulfonic acid copolymers, vinyl imidazole copolymers, vinyl sulfide copolymers, homopolymer or copolymers containing styrene sulfonic acid, and the like. The most preferred polymers are polyvinyl alcohol and its derivatives.

Other suitable polymers useful in the layer promoting adhesion to PVA include water dispersible polymers or polymer latexes. Preferably these water dispersible polymers contain at least one hydrophilic moiety, which includes hydroxyl, carboxyl, amino, or sulfonyl moieties. Such polymers include addition-type polymers and interpolymers prepared from ethylenically unsaturated monomers such as acrylates including acrylic acid, methacrylates including methacrylic acid, acrylamides and methacrylamides, itaconic acid and its half esters and diesters, styrenes including substituted styrenes, acrylonitrile and methacrylonitrile, vinyl acetates, vinyl ethers, vinyl and vinylidene halides, and olefins. In addition, crosslinking and graft-linking monomers such as 1,4-butyleneglycol methacrylate, trimethylolpropane triacrylate, allyl methacrylate, diallyl phthalate, divinyl benzene, and the like may be used. Other suitable polymer dispersions are polyurethane dispersions or polyesterionomer dispersions, polyurethane/vinyl polymer dispersions, fluoropolymer dispersions. These polymer dispersions have a particle size in the range of from 10 nanometers to 1 micron.

The layer promoting adhesion to PVA may also contain a crosslinking agent. Crosslinking agents useful for the practice of the invention include any compounds that are capable of reacting with the hydrophilic moieties attached to the polymer binder. Such crosslinking agents include aldehydes and related compounds, pyridiniums, olefins such as bis(vinylsulfonyl methyl) ether, carbodiimides, epoxides, triazines, polyfunctional aziridines, methoxyalkyl melamines, polyisocyanates, and the like. These compounds can be readily prepared using the published synthetic procedure or routine modifications that would be readily apparent to one skilled in the art of synthetic organic chemistry. Additional crosslinking agents that may also be successfully employed in the layer promoting adhesion to PVA include multivalent metal ion such as zinc, calcium, zirconium and titanium.

The layer promoting adhesion to PVA may also be an optically clear, pressure sensitive adhesive layer. A wide variety of these pressure sensitive adhesives are available. Adhesive materials useful for laminating the cover sheet to the PVA dichroic film can be selected from the general class of “modified acrylics” that have good adhesion, are transparent, and are inert with respect to chemical and UV aging and yellowing. High strength adhesives useful in this invention, for example, are aqueous-based adhesives such as Aeroset® 2177 or Aeroset®t 2550, 3240, and 3250 which are commercially available from Ashland Chemical Co., PD 0681, AP 6903, and W 3320 available from H. B. Fuller, or solvent-based pressure sensitive adhesives such as PS 508 sold by Ashland Chemical Co. The adhesives may be used separately or in combination.

The layer promoting adhesion to PVA is typically applied at a dried coating thickness of 0.1 to 5 micrometers, preferably 0.25 to 1 micrometers. The layer promoting adhesion to PVA may be on either side of the cover sheet relative to the low birefringence film. It may be adjacent to the carrier substrate or it may be on the side of the low birefringence film opposite to the carrier substrate. Preferably, the layer promoting adhesion to PVA is between the carrier substrate and the low birefringence film. Most preferably, the layer promoting adhesion to PVA is applied directly onto the carrier substrate or onto a subbing layer on the carrier substrate. The layer promoting adhesion to PVA may be coated in a separate coating application or it may be applied simultaneously with one or more other layers.

Liquid Crystal Displays typically employ two polarizer plates, one on each side of the liquid crystal cell. Each polarizer plate, in turn, employs two cover sheets, one on each side of the PVA-dichroic film. Each cover sheet may have various auxiliary layers that are necessary to improve the performance of the Liquid Crystal Display. Useful auxiliary layers employed in the cover sheets of the invention include: abrasion resistant hardcoat layer, antiglare layer, anti-smudge layer or stain-resistant layer, antireflection layer, low reflection layer, antistatic layer, viewing angle compensation layer, tie layer, and moisture barrier layer. Typically, the cover sheet closest to the viewer contains one or more of the following auxiliary layers: the abrasion resistant layer, anti-smudge or stain-resistant layer, antireflection layer, and antiglare layer. One or both of the cover sheets closest to the liquid crystal cell typically contain a viewing angle compensation layer. Any or all of the four cover sheets employed in the LCD may optionally contain tie layer, an antistatic layer and a moisture barrier layer.

In accordance with the present invention, a tie layer is advantageously employed between the layer promoting adhesion to PVA and the low birefringence polymer film to insure excellent adhesion of the hydrophilic layer promoting adhesion to PVA and the more hydrophobic low birefringence polymer film. The tie layer comprises, preferably in an amount of at least 50 weight %, of a polymer having an acid number of between 20 and 300, preferably 50 to 200. It is suitably soluble in a variety of common organic solvents at 20° C. The acid functionality is a carboxylic acid (a carboxy group, also known as a carboxyl group). Polymers suitable for use in the tie layer include copolymers (including interpolymers) of ethylenically unsaturated monomers comprising carboxylic acid groups, acid-containing cellulosic polymers such as cellulose acid phthalate and cellulose acetate trimellitate, polyurethanes having carboxylic acid groups, and others. Suitable copolymers of ethylenically unsaturated monomers comprising carboxylic acid groups include acrylates including acrylic acid, methacrylates including methacrylic acid, acrylamides and methacrylamides, itaconic acid and its half esters and diesters, styrenes including substituted styrenes, acrylonitrile and methacrylonitrile, vinyl acetates, vinyl ethers, vinyl and vinylidene halides, and olefins. Preferably, the glass transition temperature of the carboxy-functional polymer is greater than 20° C.

Organic solvents suitable for solubilizing and coating the tie layer polymer include chlorinated solvents (methylene chloride and 1,2 dichloroethane), alcohols (methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, diacetone alcohol and cyclohexanol), ketones (acetone, methylethyl ketone, methylisobutyl ketone, and cyclohexanone), esters (methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, isobutyl acetate, n-butyl acetate, and methylacetoacetate), aromatics (toluene and xylenes) and ethers (1,3-dioxolane, 1,2-dioxolane, 1,3-dioxane, 1,4-dioxane, and 1,5-dioxane). Preferably the tie-layer polymer described above is essentially soluble in at least one, preferably most of the above 28 named solvents, more preferably soluble in at least one of the solvents in most of the six mentioned groups (chlorinated, alcohols, etc.). In some applications, small amounts of water may be used. Normally, the coating solutions are prepared with a blend of the aforementioned solvents. Preferred primary solvents include methylene chloride, acetone, methyl acetate, and 1,3-dioxolane. Preferably, the tie-layer polymer is substantially soluble in these solvents. Preferred co-solvents for use with the primary solvents include methanol, ethanol, n-butanol and water. Preferably, the tie layer polymer is applied from the same or at least compatible solvent mixture as the low birefringence polymer film. In general, solubility refers to greater than 1.0 weight percent, preferably at least 2.0 percent, at 20° C.

The tie layer may also contain a crosslinking agent. Crosslinking agents useful for the practice of the invention include any compounds that are capable of reacting with reactive moieties present on the polymer, particularly carboxylic acid. Such crosslinking agents include boron-containing compounds such as borates, aldehydes and related compounds, pyridiniums, olefins such as bis(vinylsulfonyl methyl) ether, carbodiimides, polyfunctional epoxides, triazines, polyfunctional aziridines, methoxyalkyl melamines, melamine-formaldehyde resins, polyisocyanates, and the like, or mixtures thereof. These compounds can be readily prepared using the published synthetic procedure or routine modifications that would be readily apparent to one skilled in the art of synthetic organic chemistry. Additional crosslinking agents that may also be successfully employed in the layer include multivalent metal ion such as zinc, calcium, zirconium and titanium.

The tie layer is typically applied at a dried coating weight of 5 to 500 mg/ft² (50 to 5000 mg/m²), preferably 50 to 500 mg/ft² (500 to 5000 mg/m²) and has a thickness of preferably 0.5 to 5 micrometers. The layer is highly transparent and, preferably, has a light transmission of greater than 95%.

Generally, the tie layer is applied onto an already coated and dried layer promoting adhesion to PVA. The tie layer may be coated in a separate coating application or it may be applied simultaneously with one or more other layers. Preferably, for best adherence, the tie layer is applied simultaneously with the low birefringence polymer film.

Particularly effective abrasion resistant layers for use in the present invention comprise radiation or thermally cured compositions, and preferably the composition is radiation cured. Ultraviolet (UV) radiation and electron beam radiation are the most commonly employed radiation curing methods. UV curable compositions are particularly useful for creating the abrasion resistant layer of this invention and may be cured using two major types of curing chemistries, free radical chemistry and cationic chemistry. Acrylate monomers (reactive diluents) and oligomers (reactive resins and lacquers) are the primary components of the free radical based formulations, giving the cured coating most of its physical characteristics. Photo-initiators are required to absorb the UV light energy, decompose to form free radicals, and attack the acrylate group C═C double bond to initiate polymerization. Cationic chemistry utilizes cycloaliphatic epoxy resins and vinyl ether monomers as the primary components. Photo-initiators absorb the UV light to form a Lewis acid, which attacks the epoxy ring initiating polymerization. By UV curing is meant ultraviolet curing and involves the use of UV radiation of wavelengths between 280 and 420 nm preferably between 320 and 410 nm.

Examples of UV radiation curable resins and lacquers usable for the abrasion layer useful in this invention are those derived from photo polymerizable monomers and oligomers such as acrylate and methacrylate oligomers (the term “(meth)acrylate” used herein refers to acrylate and methacrylate), of polyfunctional compounds, such as polyhydric alcohols and their derivatives having (meth)acrylate functional groups such as ethoxylated trimethylolpropane tri(meth)acrylate, tripropylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, or neopentyl glycol di(meth)acrylate and mixtures thereof, and acrylate and methacrylate oligomers derived from low-molecular weight polyester resin, polyether resin, epoxy resin, polyurethane resin, alkyd resin, spiroacetal resin, epoxy acrylates, polybutadiene resin, and polythiol-polyene resin, and the like and mixtures thereof, and ionizing radiation-curable resins containing a relatively large amount of a reactive diluent. Reactive diluents usable herein include monofunctional monomers, such as ethyl (meth)acrylate, ethylhexyl (meth)acrylate, styrene, vinyltoluene, and N-vinylpyrrolidone, and polyfunctional monomers, for example, trimethylolpropane tri(meth)acrylate, hexanediol (meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, or neopentyl glycol di(meth)acrylate.

Among others, in the present invention, conveniently used radiation curable lacquers include urethane (meth)acrylate oligomers. These are derived from reacting diisocyanates with a oligo(poly)ester or oligo(poly)ether polyol to yield an isocyanate terminated urethane. Subsequently, hydroxy terminated acrylates are reacted with the terminal isocyanate groups. This acrylation provides the unsaturation to the ends of the oligomer. The aliphatic or aromatic nature of the urethane acrylate is determined by the choice of diisocyanates. An aromatic diisocyanate, such as toluene diisocyanate, will yield an aromatic urethane acrylate oligomer. An aliphatic urethane acrylate will result from the selection of an aliphatic diisocyanate, such as isophorone diisocyanate or hexyl methyl diisocyanate. Beyond the choice of isocyanate, polyol backbone plays a pivotal role in determining the performance of the final the oligomer. Polyols are generally classified as esters, ethers, or a combination of these two. The oligomer backbone is terminated by two or more acrylate or methacrylate units, which serve as reactive sites for free radical initiated polymerization. Choices among isocyanates, polyols, and acrylate or methacrylate termination units allow considerable latitude in the development of urethane acrylate oligomers. Urethane acrylates like most oligomers, are typically high in molecular weight and viscosity. These oligomers are multifunctional and contain multiple reactive sites. Because of the increased number of reactive sites, the cure rate is improved and the final product is cross-linked. The oligomer functionality can vary from 2 to 6. Among others, conveniently used radiation curable resins include polyfunctional acrylic compounds derived from polyhydric alcohols and their derivatives such as mixtures of acrylate derivatives of pentaerythritol such as pentaerythritol tetraacrylate and pentaerythritol triacrylate functionalized aliphatic urethanes derived from isophorone diisocyanate. Some examples of urethane acrylate oligomers used in the practice of this invention that are commercially available include oligomers from Sartomer Company (Exton, Pa.). An example of a resin that is conveniently used in the practice of this invention is CN968® from Sartomer Company.

A photo polymerization initiator, such as an acetophenone compound, a benzophenone compound, Michler's benzoyl benzoate, α-amyloxime ester, or a thioxanthone compound and a photosensitizer such as n-butyl amine, triethylamine, or tri-n-butyl phosphine, or a mixture thereof is incorporated in the ultraviolet radiation curing composition. In the present invention, conveniently used initiators are 1-hydroxycyclohexyl phenyl ketone and 2-methyl-1-[4-(methyl thio) phenyl]-2-morpholinopropanone-1.

The abrasion resistant layer is typically applied after coating and drying the low birefringence polymer film. The abrasion resistant layer of this invention is applied as a coating composition that typically also includes organic solvents. Preferably the concentration of organic solvent is 1-99% by weight of the total coating composition.

Examples of solvents employable for coating the abrasion resistant layer of this invention include solvents such as methanol, ethanol, propanol, butanol, cyclohexane, heptane, toluene and xylene, esters such as methyl acetate, ethyl acetate, propyl acetate and mixtures thereof. With the proper choice of solvent, adhesion of the abrasion resistant layer can be improved while minimizing migration of plasticizers and other addenda from the low birefringence polymer film, enabling the hardness of the abrasion resistant layer to be maintained. Suitable solvents for TAC low birefringence polymer film are aromatic hydrocarbon and ester solvents such as toluene and propyl acetate.

The UV polymerizable monomers and oligomers are coated and dried, and subsequently exposed to UV radiation to form an optically clear cross-linked abrasion resistant layer. The preferred UV cure dosage is between 50 and 1000 mJ/cm².

The thickness of the abrasion resistant layer is generally about 0.5 to 50 micrometers preferably 1 to 20 micrometers, more preferably 2 to 10 micrometers.

The abrasion resistant layer is preferably colorless, but it is specifically contemplated that this layer can have some color for the purposes of color correction, or for special effects, so long as it does not detrimentally affect the formation or viewing of the display through the overcoat. Thus, there can be incorporated into the polymer dyes that will impart color. In addition, additives can be incorporated into the polymer that will give to the layer desired properties. Other additional compounds may be added to the coating composition, including surfactants, emulsifiers, coating aids, lubricants, matte particles, rheology modifiers, crosslinking agents, antifoggants, inorganic fillers such as conductive and nonconductive metal oxide particles, pigments, magnetic particles, biocide, and the like.

The abrasion resistant layer of the invention typically provides a layer having a pencil hardness (using the Standard Test Method for Hardness by Pencil Test ASTM D3363) of at least 2H and preferably 2H to 8H.

The cover sheets of the invention may contain an antiglare layer, a low reflection layer or an antireflection layer on the same side of the carrier substrate as the low birefringence polymer film. Preferably, the antiglare layer, low reflection layer or antireflection layer is located on the side of the low birefringence polymer film opposite to the carrier substrate. Such layers are employed in an LCD in order to improve the viewing characteristics of the display, particularly when it is viewed in bright ambient light. The refractive index of an abrasion resistant, hard coat is about 1.50, while the index of the surrounding air is 1.00. This difference in refractive index produces a reflection from the surface of about 4%.

An antiglare coating provides a roughened or textured surface that is used to reduce specular reflection. All of the unwanted reflected light is still present, but it is scattered rather than specularly reflected. For the purpose of the present invention, the antiglare coating preferably comprises a radiation cured composition that has a textured or roughened surface obtained by the addition of organic or inorganic (matting) particles or by embossing the surface. The radiation cured compositions described hereinabove for the abrasion resistant layer are also effectively employed in the antiglare layer. Surface roughness is preferably obtained by the addition of matting particles to the radiation cured composition. Suitable particles include inorganic compounds having an oxide, nitride, sulfide or halide of a metal, metal oxides being particularly preferred. As the metal atom, Na, K, Mg, Ca, Ba, Al, Zn, Fe, Cu, Ti, Sn, In, W, Y, Sb, Mn, Ga, V, Nb, Ta, Ag, Si, B, Bi, Mo, Cei, Cd, Be, Pb and Ni are suitable, and Mg, Ca, B and Si are more preferable. An inorganic compound containing two types of metal may also be used. A particularly preferable inorganic compound is silicon dioxide, namely silica.

Additional particles suitable for use in the antiglare layer of the present invention include the layered clays described in commonly-assigned U.S. patent application Ser. No. 10/690,123, filed Oct. 21, 2003. The most suitable layered particles include materials in the shape of plates with high aspect ratio, which is the ratio of a long direction to a short direction in an asymmetric particle. Preferred layered particles are natural clays, especially natural smectite clay such as montmorillonite, nontronite, beidellite, volkonskoite, hectorite, saponite, sauconite, sobockite, stevensite, svinfordite, halloysite, magadiite, kenyaite and vermiculite as well as layered double hydroxides or hydrotalcites. Most preferred clay materials include natural montmorillonite, hectorite and hydrotalcites, because of commercial availability of these materials.

The layered materials suitable for this invention may comprise phyllosilicates, for example, montmorillonite, particularly sodium montmorillonite, magnesium montmorillonite, and/or calcium montmorillonite, nontronite, beidellite, volkonskoite, hectorite, saponite, sauconite, sobockite, stevensite, svinfordite, vermiculite, magadiite, kenyaite, talc, mica, kaolinite, and mixtures thereof Other useful layered materials may include illite, mixed layered illite/smectite minerals, such as ledikite and admixtures of illites with the layered materials named above. Other useful layered materials, particularly useful with anionic matrix polymers, may include the layered double hydroxide clays or hydrotalcites, such as Mg₆Al_(3.4)(OH)_(18.8)(CO₃)_(1.7)H₂O, which have positively charged layers and exchangeable anions in the interlayer spaces. Preferred layered materials are swellable so that other agents, usually organic ions or molecules, may splay, that is, intercalate and/or exfoliate, the layered material resulting in a desirable dispersion of the inorganic phase. These swellable layered materials include phyllosilicates of the 2:1 type, as defined in the literature (for example, “An introduction to clay colloid chemistry,” by H. van Olphen, John Wiley & Sons Publishers). Typical phyllosilicates with ion exchange capacity of 50 to 300 milliequivalents per 100 grams are preferred. Generally, it is desirable to treat the selected clay material to separate the agglomerates of platelet particles to small crystals, also called tactoids, prior to introducing the platelet particles to the antiglare coating. Predispersing or separating the platelet particles also improves the binder/platelet interface. Any treatment that achieves the above goals may be used. Examples of useful treatments include intercalation with water soluble or water insoluble polymers, organic reagents or monomers, silane compounds, metals or organometallics, organic cations to effect cation exchange, and their combinations.

Additional particles for use in the antiglare layer of the present invention include polymer matte particles or beads which are well known in the art. The polymer particles may be solid or porous, preferably they are crosslinked polymer particles. Porous polymer particles for use in an antiglare layer are described in commonly-assigned U.S. patent application Ser. No. 10/715,706, filed Nov. 18, 2003.

Particles for use in the antiglare layer have an average particle size ranging from 2 to 20 micrometers, preferably from 2 to 15 micrometers and most preferably from 4 to 10 micrometers. They are present in the layer in an amount of at least 2 wt percent and less than 50 percent, typically from about 2 to 40 wt. percent, preferably from 2 to 20 percent and most preferably from 2 to 10 percent.

The thickness of the antiglare layer is generally about 0.5 to 50 micrometers preferably 1 to 20 micrometers more preferably 2 to 10 micrometers.

Preferably, the antiglare layer used in the present invention has a 60° Gloss value, according to ASTM D523, of less than 100, preferably less than 90 and a transmission haze value, according to ASTM D-1003 and JIS K-7105 methods, of less than 50%, preferably less than 30%.

In another embodiment of the present invention, a low reflection layer or antireflection layer is used in combination with an abrasion resistant hard coat layer or antiglare layer. The low reflection or antireflection coating is applied on top of the abrasion resistant or antiglare layer. Typically, a low reflection layer provides an average specular reflectance (as measured by a spectrophotometer and averaged over the wavelength range of 450 to 650 nm) of less than 2%. Antireflection layers provide average specular reflectance values of less than 1%.

Suitable low reflection layers for use in the present invention comprise fluorine-containing homopolymers or copolymers having a refractive index of less than 1.48, preferably with a refractive index between about 1.35 and 1.40. Suitable fluorine-containing homopolymers and copolymers include: fluoro-olefins (for example, fluoroethylene, vinylidene fluoride, tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoro-2,2-dimethyl-1,3-dioxol), partially or completely fluorinated alkyl ester derivatives of (meth)acrylic acid, and completely or partially fluorinated vinyl ethers, and the like. The effectiveness of the layer may be improved by the incorporation of submicron-sized inorganic particles or polymer particles that induce interstitial air voids within the coating. This technique is further described in U.S. Pat. No. 6,210,858 and U.S. Pat. No. 5,919,555. Further improvement of the effectiveness of the low reflection layer may be realized with the restriction of air voids to the internal particle space of submicron-sized polymer particles with reduced coating haze penalty, as described in commonly-assigned U.S. patent application Ser. No. 10/715,655, filed Nov. 18, 2003.

The thickness of the low reflection layer is 0.01 to 1 micrometer and preferably 0.05 to 0.2 micrometer.

An antireflection layer may comprise a monolayer or a multi-layer. Antireflection layers comprising a monolayer typically provide reflectance values less than 1% at only a single wavelength (within the broader range of 450 to 650 nm). A commonly employed monolayer antireflection coating that is suitable for use in the present invention comprises a layer of a metal fluoride such as magnesium fluoride (MgF₂). The layer may be applied by well-known vacuum deposition technique or by a sol-gel technique. Typically, such a layer has an optical thickness (i.e., the product of refractive index of the layer times layer thickness) of approximately one quarter-wavelength at the wavelength where a reflectance minimum is desired.

Although a monolayer can effectively reduce the reflection of light within a very narrow wavelength range, more often a multi-layer comprising several (typically, metal oxide based) transparent layers superimposed on one another is used to reduce reflection over a wide wavelength region (i.e., broadband reflection control). For such a structure, half wavelength layers are alternated with quarter wavelength layers to improve performance. The multi-layer antireflection coating may comprise two, three, four, or even more layers. Formation of this multi-layer typically requires a complicated process comprising a number of vapor deposition procedures or sol-gel coatings, which correspond to the number of layers, each layer having a predetermined refractive index and thickness. Precise control of the thickness of each layer is required for these interference layers. The design of suitable multi-layer antireflection coatings for use in the present invention is well known in the patent art and technical literature, as well as being described in various textbooks, for example, in H. A. Macleod, “Thin Film Optical Filters,” Adam Hilger, Ltd., Bristol 1985 and James D. Rancourt, “Optical Thin Films User's Handbook”, Macmillan Publishing Company, 1987.

The cover sheets of the invention may contain a moisture barrier on one or both sides of the low birefringence polymer film. The moisture barrier layer comprises a hydrophobic polymer such as a vinylidene chloride polymer, vinylidene fluoride polymer, polyurethane, polyolefin, fluorinated polyolefin, polycarbonate, and others, having a low moisture permeability. Preferably, the hydrophobic polymer comprises vinylidene chloride. More preferably, the hydrophobic polymer comprises 70 to 99 weight percent of vinylidene chloride. The moisture barrier layer may be applied by application of an organic solvent-based or aqueous coating formulation. To provide effective moisture barrier properties the layer should be at least 1 micrometer in thickness, preferably from 1 to 10 micrometers in thickness, and most preferably from 2 to 8 micrometers in thickness. The cover sheet of the invention comprising a moisture barrier layer has a moisture vapor transmission rate (MVTR) according to ASTM F-1249 that is less than 1000 g/m²/day, preferably less than 800 g/m²/day and most preferably less than 500 g/m²/day. The use of such a barrier layer in the cover sheet of the invention provides improved resistance to changes in humidity and increased durability of the polarizer comprising the cover sheet, especially for TAC cover sheets having a thickness less than about 40 micrometers.

The cover sheets of the invention may contain a transparent antistatic layer on either side of the low birefringence polymer film. The antistatic layer aids in the control of static charging that may occur during the manufacture and use of the cover sheet composite. Effective control of static charging reduces the propensity for the attraction of dirt and dust to the cover sheet composite. The guarded cover sheet composite of the invention may be particularly prone to triboelectric charging during the peeling of the cover sheet from the carrier substrate. The so-called “separation charge” that results from the separation of the cover sheet and the substrate can be effectively controlled by an antistatic layer having a resistivity of less than about 1×10¹¹ Ω/square, preferably less than 1×10¹⁰ Ω/square, and most preferably less than 1×10⁹ Ω/square.

Various polymeric binders and conductive materials may be employed in the antistatic layer. Polymeric binders useful in the antistatic layer include any of the polymers commonly used in the coating art, for example, interpolymers of ethylenically unsaturated monomers, cellulose derivatives, polyurethanes, polyesters, hydrophilic colloids such as gelatin, polyvinyl alcohol, polyvinyl pyrrolidone, and others.

Conductive materials employed in the antistatic layer may be either ionically-conductive or electronically-conductive. Ionically-conductive materials include simple inorganic salts, alkali metal salts of surfactants, polymeric electrolytes containing alkali metal salts, and colloidal metal oxide sols (stabilized by metal salts). Of these, ionically-conductive polymers such as anionic alkali metal salts of styrene sulfonic acid copolymers and cationic quaternary ammonium polymers of U.S. Pat. No. 4,070,189 and ionically-conductive colloidal metal oxide sols which include silica, tin oxide, titania, antimony oxide, zirconium oxide, alumina-coated silica, alumina, boehmite, and smectite clays are preferred.

The antistatic layer employed in the current invention preferably contains an electronically-conductive material due to their humidity and temperature independent conductivity. Suitable materials include:

1) electronically-conductive metal-containing particles including donor-doped metal oxides, metal oxides containing oxygen deficiencies, and conductive nitrides, carbides, and bromides. Specific examples of particularly useful particles include conductive SnO₂, In₂O, ZnSb₂O₆, InSbO₄, TiB₂, ZrB₂, NbB₂, TaB₂, CrB, MoB, WB, LaB₆, ZrN, TiN, WC, HfC, HfN, and ZrC. Examples of the patents describing these electrically conductive particles include; U.S. Pat. Nos. 4,275,103; 4,394,441; 4,416,963; 4,418,141; 4,431,764; 4,495,276; 4,571,361; 4,999,276; 5,122,445; and 5,368,995. 2) fibrous electronic conductive particles comprising, for example, antimony-doped tin oxide coated onto non-conductive potassium titanate whiskers as described in U.S. Pat. Nos. 4,845,369 and 5,166,666, antimony-doped tin oxide fibers or whiskers as described in U.S. Pat. Nos. 5,719,016 and 5,0731,119, and the silver-doped vanadium pentoxide fibers described in U.S. Pat. No. 4,203,769 3) electronically-conductive polyacetylenes, polythiophenes, and polypyrroles, preferably the polyethylene dioxythiophene described in U.S. Pat. No. 5,370,981 and commercially available from Bayer Corp. as Baytron® P.

The amount of the conductive agent used in the antistatic layer of the invention can vary widely depending on the conductive agent employed. For example, useful amounts range from about 0.5 mg/m² to about 1000 mg/m², preferably from about 1 mg/m² to about 500 mg/m². The antistatic layer has a thickness of from 0.05 to 5 micrometers, preferably from 0.1 to 0.5 micrometers to insure high transparency.

Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays, which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display is the propensity for light to “leak” through liquid crystal elements or cells, which are in the dark or “black” pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the direction from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle range centered about the normal incidence to the display and falls off rapidly as the viewing direction deviates from the display normal. In color displays, the leakage problem not only degrades the contrast but also causes color or hue shifts with an associated degradation of color reproduction.

Thus, one of the major factors measuring the quality of LCDs is the viewing angle characteristic, which describes a change in contrast ratio from different viewing angles. It is desirable to be able to see the same image from a wide variation in viewing angles and this ability has been a shortcoming with liquid crystal display devices. One way to improve the viewing angle characteristic is to employ a cover sheet having a viewing angle compensation layer (also referred to as a compensation layer, retarder layer, or phase difference layer), with proper optical properties, between the PVA-dichroic film and liquid crystal cell, such as disclosed in U.S. Pat. Nos. 5,583,679, 5,853,801, 5,619,352, 5,978,055, and 6,160,597. A compensation film according to U.S. Pat. Nos. 5,583,679 and 5,853,801 based on discotic liquid crystals which have negative birefringence, is widely used.

Viewing angle compensation layers useful in the present invention are optically anisotropic layers. The optically anisotropic, viewing angle compensation layers may comprise positively birefringent materials or negatively birefringent materials. The compensation layer may be optically uniaxial or optically biaxial. The compensation layer may have its optic axis tilted in the plane perpendicular to the layer. The tilt of the optic axis may be constant in the layer thickness direction or the tilt of the optic axis may vary in the layer thickness direction.

Optically anisotropic, viewing angle compensation layers useful in the present invention may comprise the negatively birefringent, discotic liquid crystals described in U.S. Pat. Nos. 5,583,679, and 5,853,801; the positively birefringent nematic liquid crystals described in U.S. Pat. No. 6,160,597; the negatively birefringent amorphous polymers described in commonly assigned U.S. Patent Application Publication 2004/0021814A and U.S. patent application Ser. No. 10/745,109, filed Dec. 23, 2003. These latter two patent applications describe compensation layers comprising polymers that contain non-visible chromophore groups such as vinyl, carbonyl, amide, imide, ester, carbonate, sulfone, azo, and aromatic groups (i.e. benzene, naphthalate, biphenyl, bisphenol A) in the polymer backbone and that preferably have a glass transition temperature of greater than 180 degree C. Such polymers are particularly useful in the compensation layer of the present invention. Such polymers include polyesters, polycarbonates, polyimides, polyetherimides, and polythiophenes. Of these, particularly preferred polymers for use in the present invention include: (1) a poly(4,4′-hexafluoroisopropylidene-bisphenol) terephthalate-co-isophthalate, (2) a poly(4,4′-hexahydro-4,7-methanoindan-5-ylidene bisphenol) terephthalate, (3) a poly(4,4′-isopropylidene-2,2′6,6′-tetrachlorobisphenol) terephthalate-co-isophthalate, (4) a poly(4,4′-hexafluoroisopropylidene)-bisphenol-co-(2-norbornylidene)-bisphenol terephthalate, (5) a poly(4,4′-hexahydro-4,7-methanoindan-5-ylidene)-bisphenol-co-(4,4′-isopropylidene-2,2′,6,6′-tetrabromo)-bisphenol terephthalate, (6) a poly(4,4′-isopropylidene-bisphenol-co-4,4′-(2-norbornylidene) bisphenol) terephthalate-co-isophthalate, (7) a poly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene) bisphenol) terephthalate-co-isophthalate, or (8) copolymers of any two or more of the foregoing. A compensation layer comprising these polymers typically has an out-of-plane retardation, R_(th), that is more negative than −20 nm; preferably R_(th) is from −60 to −600 nm, and most preferably R_(th) is from −150 to −500 nm.

Another compensation layer suitable for the present invention includes an optically anisotropic layer comprising an exfoliated inorganic clay material in a polymeric binder as described in Japanese Patent Application 11095208A.

The auxiliary layers of the invention can be applied by any of a number of well known liquid coating techniques, such as dip coating, rod coating, blade coating, air knife coating, gravure coating, microgravure coating, reverse roll coating, slot coating, extrusion coating, slide coating, curtain coating, or by vacuum deposition techniques. In the case of liquid coating, the wet layer is generally dried by simple evaporation, which may be accelerated by known techniques such as convection heating. The auxiliary layer may be applied simultaneously with other layers such as subbing layers and the low birefringence polymer film. Several different auxiliary layers may be coated simultaneously using slide coating, for example, an antistatic layer may be coated simultaneously with a moisture barrier layer or a moisture barrier layer may be coated simultaneously with a viewing angle compensation layer. Known coating and drying methods are described in further detail in Research Disclosure 308119, Published Dec. 1989, pages 1007 to 1008.

The cover sheets of the invention are suitable for use with a wide variety of LCD display modes, for example, Twisted Nematic (TN), Super Twisted Nematic (STN), Optically Compensated Bend (OCB), In Plane Switching (IPS), or Vertically Aligned (VA) liquid crystal displays. These various liquid crystal display technologies have been reviewed in U.S. Pat. Nos. 5,619,352 (Koch et al.), 5,410,422 (Bos), and 4,701,028 (Clerc et al.).

As should be obvious based on the preceding detailed description, a wide variety of guarded cover sheet composites having various types and arrangements of auxiliary layers may be prepared. Some of the configurations possible in accordance with the present invention are illustrated by the following non-limiting examples.

Guarded Cover Sheet Composite C1:

TAC PVA layer carrier substrate PVA layer TAC A 60 micrometer thick polyethylene terephthalate carrier substrate is coated on each side with a layer promoting adhesion to PVA comprising the polyvinyl alcohol polymer Cervol® 107 PVA (98-99% hydrolyzed, available from Celanese Corp.) that has a dried thickness of 1 micrometer. Each dried PVA layer is then overcoated with a triacetyl cellulose (TAC) formulation in a sequential operation. The dried TAC layers are 20 micrometers in thickness and contain 11 wt % triphenyl phosphate plasticizer, 1 wt % TINUVIN® 8515 UV absorber (a mixture of 2-(2′-Hydroxy-3′-tert-butyl-5′-methylphenyl)-5-chloro benzotriazole and 2-(2′-Hydroxy-3′,5′-ditert-butylphenyl)-benzotriazole, available from Ciba Specialty Chemicals) and about 0.1 wt % PARSOL® 1789 UV absorber (4-(1,1-dimethylethyl)-4′-methoxydibenzoylmethane, available from Roche Vitamins Inc.). Guarded Cover Sheet Composite C2:

PVA layer TAC carrier substrate TAC PVA layer Guarded Cover Sheet Composite C2 is prepared in an analogous manner to composite C1 except that the TAC layer are applied on the carrier substrate and the PVA layers are applied onto the dried TAC layers. Guarded Cover Sheet Composite C3:

TAC release layer primer layer carrier substrate primer layer release layer TAC Guarded Cover Sheet Composite C3 is prepared in an analogous manner to composite C1 except on each side of the polyethylene terephthalate carrier substrate there is a first subbing layer that is a 0.1 micrometer thick primer layer comprising poly(vinylidene chloride-co-acrylonitrile-co-acrylic acid) and a second subbing layer that is a 0.5 micrometer thick release layer comprising polyvinyl butyral. Guarded Cover Sheet Composite C4:

abrasion resistant layer antistatic layer TAC tie layer PVA layer carrier substrate PVA layer tie layer TAC Guarded cover sheet composite C4 is prepared in an analogous manner to composite C1 except that 1 micrometer thick tie layers comprising a 87/8.7/4.3 mixture of Carboset® 525 carboxylated acrylic polymer (Noveon Inc.), Desmodur N100 isocyanate (Bayer Materials Science AG) and trimethyl borate are coated and dried simultaneously with the TAC layers. An antistatic layer comprising Baytron® P (polyethylene dioxythiophene/polystyrene sulfonate, available from Bayer Corp) in a poly(vinylidene chloride-co-acrylonitrile-co-acrylic acid) binder is applied onto the TAC layer on one side of the carrier substrate. The antistatic layer contains 3 mg/m² Baytron® P and has a surface resistivity of about 1×10⁸ Ω/square. An abrasion resistant layer is applied onto the antistatic layer by coating, drying and then UV curing a urethane acrylate oligomer, CN968® from Sartomer Company. Guarded Cover Sheet Composite C5:

abrasion resistant layer TAC carrier substrate cyclic olefin polymer viewing angle compensation layer A 60 micrometer thick polyethylene terephthalate carrier substrate is coated on one side with a triacetyl cellulose (TAC) formulation to give a dried TAC layer that is 20 micrometers in thickness and contains 11 wt % triphenyl phosphate plasticizer, 1 wt % TINUVIN® 8515 UV absorber (a mixture of 2-(2′-Hydroxy-3′-tert-butyl-5′-methylphenyl)-5-chloro benzotriazole and 2-(2′-Hydroxy-3′,5′-ditert-butylphenyl)-benzotriazole, available from Ciba Specialty Chemicals) and about 0.1 wt % PARSOL® 1789 UV absorber (4-(1,1-dimethylethyl)-4′-methoxydibenzoylmethane, available from Roche Vitamins Inc.). The other side of the carrier substrate is coated with 20 micrometer thick layer of a cyclic olefin polymer, Zeonor® available from Nippon Zeon. An abrasion resistant layer is applied onto the TAC film by coating, drying and then UV curing a urethane acrylate oligomer, CN968® from Sartomer Company. Finally, a viewing angle compensation layer is applied onto the cyclic olefin polymer film. The viewing angle compensation layer is a 3 micrometer thick layer comprising poly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene) bisphenol) terephthalate-co-isophthalate. Guarded Cover Sheet Composite C6:

abrasion resistant layer antistatic layer TAC tie layer PVA layer carrier substrate PVA layer tie layer TAC Guarded cover sheet composite C6 is prepared in an analogous manner to composite C4 except that the TAC layer on the same side of the carrier substrate as the abrasion resistant layer is only 10 micrometers in thickness and the TAC layer on the opposite side is 20 micrometers in order to minimize the curl of the guarded cover sheet composite.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

PARTS LIST

-   5 tandem coating and drying system -   8 simultaneous coating and drying system -   10 coating and drying apparatus -   12 moving substrate -   14 dryer -   14 a dryer -   14 b dryer -   15 web conveyance element -   16 coating apparatus -   16 a coating apparatus -   16 b coating apparatus -   16 c coating apparatus -   16 d coating apparatus -   18 unwinding station -   20 back-up roller -   20 a back-up roller -   20 b back-up roller -   22 coated substrate -   22 a coated substrate -   22 b coated substrate -   22 c coated substrate -   24 guarded cover sheet composite -   26 winding station -   28 coating supply vessel -   28 a coating supply vessel -   28 b coating supply vessel -   28 c coating supply vessel -   28 d coating supply vessel -   30 coating supply vessel -   30 a coating supply vessel -   30 b coating supply vessel -   32 coating supply vessel -   32 a coating supply vessel -   32 b coating supply vessel -   34 coating supply vessel -   34 a coating supply vessel -   34 b coating supply vessel -   36 pumps -   36 a pumps -   36 b pumps -   36 c pumps -   36 d pumps -   38 pumps -   38 a pumps -   38 b pumps -   40 pumps -   40 a pumps -   40 b pumps -   42 pumps -   42 a pumps -   42 b pumps -   44 conduits -   44 a conduits -   44 b conduits -   46 conduits -   46 a conduits -   46 b conduits -   48 conduits -   48 a conduits -   48 b conduits -   50 conduits -   50 a conduits -   50 b conduits -   52 discharge device -   52 a discharge device -   52 b discharge device -   54 polar charge assist device -   54 a polar charge assist device -   54 b polar charge assist device -   66 drying section -   68 drying section -   70 drying section -   72 drying section -   74 drying section -   76 drying section -   78 drying section -   80 drying section -   82 drying section -   92 front section -   94 second section -   96 third section -   98 fourth section -   100 back plate -   102 inlet -   104 metering slot -   106 pump -   108 lower most layer -   110 inlet -   112 2^(nd) metering slot -   114 pump -   116 layer -   118 inlet -   120 metering slot -   122 pump -   124 form layer inlet -   126 inlet -   128 metering slot -   130 pump -   132 layer -   134 incline slide surface -   136 coating lip -   138 2^(nd) incline slide surface -   140 3^(rd) incline slide surface -   142 4^(th) incline slide surface -   144 back land surface -   146 coating bead -   151 guarded cover sheet composite -   153 guarded cover sheet composite -   155 guarded cover sheet composite -   161 lowermost layer -   161 a lowermost layer -   161 b lowermost layer -   162 lowermost layer -   162 a lowermost layer -   162 b lowermost layer -   163 intermediate layer -   164 intermediate layer -   165 outermost layer -   165 a outermost layer -   165 b outermost layer -   166 outermost layer -   166 a outermost layer -   166 b outermost layer -   170 carrier support -   171 cover sheet -   173 cover sheet -   175 cover sheet -   177 cover sheet -   179 cover sheet -   181 cover sheet -   182 carrier substrate -   184 release layer -   200 feed line -   202 extrusion hopper -   204 pressurized tank -   206 pump -   208 metal drum -   210 drying section -   212 drying oven -   214 cast film -   216 final drying section -   218 final dried film -   220 wind-up station -   232 guarded cover sheet composite supply roll -   236 PVA-dichroic film supply roll -   238 PVA-dichroic film -   240 carrier substrate take-up roll -   242 opposing pinch roll -   244 opposing pinch roll -   250 polarizer plate -   252 polarizer plate -   254 polarizer plate -   260 LCD cell -   261 layer promoting adhesion to PVA -   262 low birefringence polymer film -   264 moisture barrier layer -   266 antistatic layer -   268 antiglare layer -   272 viewing angle compensation layer 

1. A guarded cover sheet composite comprising a temporary carrier substrate having a first cover sheet comprising a first low birefringence film on one side and a second cover sheet comprising a second low birefringence film on the other side, wherein the low birefringence films have an in-plane birefringence, Δn_(in) of less than about 1×10⁻⁴ and an out-of-plane birefringence, Δn_(th) of from 0.005 to −0.005.
 2. The composite of claim 1 wherein said first and/or second low birefringence polymer films comprise cellulose esters.
 3. The composite of claim 1 wherein said first and second low birefringence polymer films comprise the same material.
 4. The composite of claim 1 wherein both cover sheets further comprise a layer promoting adhesion to polyvinyl alcohol.
 5. The composite of claim 4 wherein said layer promoting adhesion to polyvinyl alcohol is adjacent to said carrier substrate.
 6. The composite of claim 4 wherein said layer promoting adhesion to polyvinyl alcohol comprises hydrophilic polymer.
 7. The composite of claim 1 wherein at least one cover sheet further comprises an auxiliary layer.
 8. The composite of claim 7 wherein said auxiliary layer is an abrasion resistant layer, a low reflection layer, an antireflection layer, an antiglare layer, a compensation layer, an antistatic layer, a tie layer or a barrier layer.
 9. The composite of claim 8 wherein said auxiliary layer is an abrasion resistant layer.
 10. The composite of claim 9 wherein said abrasion resistant layer comprises a radiation cured acrylic polymer. 